The present invention relates to an ultrafine grinding mill of which fed material (raw material to be ground) flows down through an agitated bed composed of small grinding medium and more particularly to such an ultrafine grinding mill which can produce spherical ultrafine particles each having a diameter less than about 2 .mu.m by a dry process. Such spherical ultrafine particle of a diameter less than 2 .mu.m is usually used, from its configuration characteristics, for packing material, coating material for papermaking, pigment, filler and other materials required for an interfacial control of high accuracy.
It is known in prior art several kinds of ultrafine grinding mills such as a mill having a hummer or rotor of high rate of revolution whose fed material is ground by impact and shearing, a ball mill whose fed material is ground by mutual collision between balls, a jet grinding mill whose fed material is ground by mutual collision between jet flows including fed material, and a medium agitation mill in which a mixture of fed material and grinding medium is agitated and ground by abrasion. In these ultrafine grinding mills, the medium agitation mill is adapted to produce ultrafine particles by mutual abrasion between grinding medium particles and fed material particles and is suited for forming powder of submicron range in which the plastic breakage is at advantage over the elastic breakage.
In prior art ultrafine grinding mills of dry type, any mill causes both a grinding action of raw material to be ground (fed material) and an agglomerating phenomenon what is called negative grinding, due to the recombination of ground product (material formed by grinding) within a mill. Accordingly the particle size of fineness limit (i.e. grinding limit achieved by grinding) is determined by an equilibrium state between the grinding rate and the agglomerating rate. This agglomerating phenomenon is particularly remarkable in the ball mill, the vibration ball mill and the planetary mill in which the fed material is ground by the impact action of the grinding medium. The grinding medium, on the one hand, accelerates the grinding of the fed material and on the other hand, accelerates the agglomerating phenomenon due to the pressure adhesion of newly ground product.
In order to quickly discharge the ground product from the grinding mill so as to prevent the agglomerating phenomenon, there has been used a method of a type "airflow discharge/separetely installed classifier" and there has been proposed a grinding mill of a type "airflow discharge/built-in classifier". However it is difficult to perfectly disperse a group of ground particle products having a max. particle size (i.e. a top size) of few .mu.m's due to influences of moisture or static electricity and also it is difficult to apply said method and grinding mill for ultrafine grinding because the action based upon the settling velocity and body force (volume force) in an airflow of particles contributing to the classifying action drastically decreases in proportion to (particle size) .sup.-3.